1. Technical Field
The present invention is directed to a process for device fabrication in which the process is controlled by inspecting the latent features introduced into an energy sensitive resist material.
2. Art Background
The advent of semiconductor devices and structures with submicron device rules has presented a need for more rigorous process control. In lithographic processes employed as a step in device fabrication (i.e. processes in which a desired pattern is introduced into an energy sensitive resist material via a patternwise exposure to energy, developed, and transferred into an underlying substrate) the process is controlled by generating a sample of the pattern, inspecting the pattern that is actually formed in the energy sensitive resist material, comparing the actual pattern in the sample to the desired pattern, and making the necessary adjustments to the process to make the actual pattern correspond more closely to the desired pattern. The problem associated with this technique is determining the parameter that must be adjusted so that the actual pattern corresponds acceptably to the desired pattern. The information obtained by inspecting the developed resist features does not provide a basis for real-time control of the lithographic process.
In lithographic processes, a discrepancy between the actual pattern and the desired pattern can result from a problem with the mask (i.e. the patterned reticle used to patternwise expose the energy sensitive resist material), a problem with the apparatus used to introduce the image in the energy sensitive resist material, or a problem with other materials and equipment used in the lithographic process. The act of inspecting the developed resist features does not pinpoint the source of the error if a greater-than-acceptable discrepancy is discovered between the desired pattern and the actual pattern.
In response to this problem, techniques have been developed for inspecting the latent features introduced into an energy sensitive material before that image is developed. One such technique is known as latent image metrology. By detecting a discrepancy between the latent image of the actual pattern and the desired pattern, one can more quickly pinpoint the source of that discrepancy. One such technique is described in Adams, T. E., "Applications of Latent Image Metrology in Microlithography", Proc. SPIE 1464, p. 294 (1991).
Adams describes a technique in which a latent image is introduced into an energy sensitive resist material and a portion of that latent image is subjected to white light which is scattered by the energy sensitive resist material. The latent image in the resist material affects the light in a way that provides information about the latent image. The scattered light is then detected. The amplitude of the detected signal is then used to evaluate the lithographic apparatus design, or adjust the exposure dose or focus used to introduce the image into the resist material. This evaluation is accomplished by analyzing the scattering amplitude as a function of the various image printing parameters.
This technique for evaluating a latent feature is limited because the classical diffraction limit for light is typically greater than 0.5 .mu.m. Therefore, images of feature sizes that are less than 0.5 .mu.m in size are difficult to detect accurately with this technique. Furthermore, to use this technique, a periodic array of features must be printed in the resist material. In a blanket exposure, however, the entire resist layer is exposed without features. This technique cannot be used to obtain meaningful information concerning a blanket exposure because there is an insufficient difference in scattering amplitude in a blanket exposure without features. Subtle differences in the blanket exposure caused by an uneven exposure or a less than uniform resist material will not be detected if a blanket exposure is examined using this technique.